In recent years, it becomes more often that digitally modulated signals are transmitted at transmission apparatuses employed in wireless communication and broadcasting. These signals are now able to carry information in an amplitude direction due to introduction of M-ary techniques, and linearity is therefore required in an amplifier circuit employed in a transmission apparatus. On the other hand, high power efficiency is required to an amplifier circuit in order to curtail the power consumption of an apparatus. Various techniques have been proposed for distortion correction and efficiency improvement, in order to pursue both of the linearity and power efficiency of the amplifier circuit. There exists one of conventional systems for the amplifier circuit, which is referred to as LINC (Linear Amplification with Nonlinear Components) system. In this system, a transmission signal is divided into two constant-envelope signals, and is synthesized after being amplified at a non-linear amplifier of high power efficiency, so as to pursue both linearity and transmission efficiency.
Here, a description will be given using FIG. 1 of a typical example of an amplifier circuit to which the LINC system is applied. With an amplifier circuit 10 shown in FIG. 1, at a constant-envelope signal generating section 11, two constant-envelope signals Sa(t) and Sb(t) are generated from an input signal S(t). For example, if each constant-envelope signal Sa(t) and Sb(t) is assumed to be given by the following (equation 2) and (equation 3) when the input signal S(t) is represented by the following (equation 1), then each constant-envelope signal Sa(t) and Sb(t) is a constant value in its amplitude direction.S(t)=V(t)×cos {ωct+φ(t)}  (Equation 1)Here, the maximum value for V(t) is assumed to be Vmax, and the angular frequency of the carrier for the input signal is assumed to be ωc.Sa(t)=Vmax/2×cos {ωct+ψ(t)}  (Equation 2)Sb(t)=Vmax/2×cos {ωct+θ(t)}  (Equation 3)where ψ(t)=φ(t)+α(t) and θ(t)=φ(t)−α(t).
In FIG. 2, the operation of generating the constant-envelope signals is shown using signal vectors on coordinates in an orthogonal plane, and as shown in this figure, the input signal S(t) is represented as the vector sum of two constant-envelope signals Sa(t) and Sb(t) of which amplitude is Vmax/2.
Referring again to FIG. 1, two constant-envelope signals are respectively amplified by two amplifiers 12 and 13. At this time, assuming the gain of amplifiers 12 and 13 to be G, output signals of amplifiers 12 and 13 are G×Sa(t) and G×Sb(t), respectively. At combining section 14, when the output signals G×Sa(t) and G×Sb(t) are combined, an output signal G×S(t) is obtained.
An example of an amplifier circuit 10a having a similar configuration to this is shown in FIG. 3. In constant-envelope signal generating section 11, baseband signals Sai, Saq, Sbi and Sbq, which constitute constant-envelope signals Sa and Sb after orthogonal demodulation from baseband input signals Si and Sq, are generated by digital signal processing at constant-envelope signal IQ generating section 15. After these baseband signals are converted to analogue signals using D/A converters 16a, 16b, 16c and 16d, the signals are orthogonally modulated by orthogonal modulator 17 having two orthogonal modulators so as to obtain two constant-envelope signals Sa(t) and Sb(t). After each signal is amplified by amplifiers (driver amplifiers) 18a and 18b, final amplification by means of the amplifiers 12 and 13 and combining by means of combining section 14 are carried out, which results in obtainment of an output signal.
At amplifier circuit 10a as described above, constant-envelope signal generation can be achieved with digital signal processing by employing baseband signals of low frequency. However, in the event that errors occur in the gain or phase of the amplifiers in the two systems, vectors of signals after amplification and combining may differ from vectors of an intended output signal, i.e., these errors may become distortion components of signals. Further, at the amplifier circuit 10a, not only is it difficult to predict factors for these errors, but characteristics fluctuate due to an environment such as temperature or the like.
In order to correct this in a conventional amplifier circuit, a method is proposed (for example, refer to Patent Document 1) where, for example, an auxiliary wave signal combined with an input signal upon generation of constant-envelope signals is approximated, two constant-envelope signals are generated by combining the auxiliary wave signal and input signal, the constant-envelope signals are amplified with two amplifiers and combined, and then an output signal or auxiliary wave component is detected to correct errors in characteristics related to the gain or phase of the amplifiers in the two systems.
Patent Document 1: Japanese Patent Publication No. 2758682